1. Field of the Invention
The present invention relates to a system for detecting an off-track amount of a present head position with respect to the normal position of a desired track in a disc storage unit and especially in a hard disc storage unit of the type in which a so-called closed loop control is employed to control the position of an information read/write transducer such as a magnetic head.
2. Description of the Prior Art
A hard disc unit which typically represents a disc storage unit of the type described above is provided with one or more magnetic discs as media for storing information as is well known in the art and these discs are rotated normally at a high rotational speed on the order of thousands of r.p.m. by a spindle motor. Each disc has a plurality of concentric tracks for storing information. In order to write in and read out information to and from a surface of the disc respectively, one read/write head is assigned to each disc surface and is properly positioned on a desired track of a plurality of tracks and in principle is positioned at the center of the desired track.
There has been an increasing demand for a disc storage unit which is compact in size and yet capable of storing information at a high data storage density. That is, the diameter of the disc is reduced, while the data storage density is greatly increased.
In the case of a relatively low data storage density, an open loop control has long been used to control the position of the read/write head. According to this control system, only a predetermined number of drive command pulses are applied to an actuator such as a pulse motor for rotating the read/write head, so that the head is moved toward a track of a selected number, but the actual position of the read/write head is not detected. As a result, a fine adjustment of the position of the read/write head is not accomplished. It has been reported that the maximum number of tracks which can be selected by such an open loop control system is 500 TPI (track per inch), for example, in the case of a hard disc of 5.25 inches in diameter.
If the data storage density is increased and exceeds 700 TPI, it is required to employ a closed loop system in order to control the position of the read/write head. A closed loop control system is based upon correct detection of the position of the read/write head, and hence various devices have been made for such a detection. In principle, the most correct and simplest manner of head position detection is such that instead of mounting a special position sensor on the read/write head, reference information for detecting the position of the read/write head is previously written into a disc and is read out by the read/write head itself whose position is to be detected, so that the position of the read/write head is detected in response to the read out reference information.
A closed loop control system of the type described above is disclosed in detail in, for instance, Japanese Patent Application Laying-open No. 50-99709. In the prior art closed loop control system of the type described above, a reference information pattern to be recorded on the disc or a method for processing the read out reference information has been devised, but in these cases there is a common defect in that a complicated and highly sophisticated signal processing circuit is required to detect the position of the read/write head.
More particularly, according to a closed loop control system of the type described above, reference information is in the form of a magnetic pattern of N and S poles, which is written in such a way that boundaries between the N and S poles are displaced from each other between adjacent reference information. Adjacent reference information data are distinguished from each other by utilizing the deviation between pulse generating timings in a plurality of groups each consisting of a plurality of analog pulses which have their peak values at the boundaries between N and N poles in signals read out by the read/write head. In response to the difference in peak values between the analog pulses, the off-track amount between the present position of the read/write head and the correct position thereof is detected.
It follows, therefore, that reference information must be written into a disc with a high degree of accuracy so that analog pulses which belong to different reference information areas can be distinctly and sharply separated from each other. Furthermore, the peak values of analog pulses belonging to adjacent reference information areas must be temporarily and correctly stored in an analog manner in a peak hold circuit and then their difference must be detected correctly. As a result, a highly sophisticated analog-value peak-hold circuit and a highly sophisticated arithmetic circuit must be employed and it is unavoidable that their associated circuits become complicated.
At an initial stage where such a closed control system inclusive of the type described above is introduced, the position of the read/write head is always controlled, while the head is reading or writing specific information from or in a surface of the disc, so that the region for reference information which is used for the detection of the read/write head must be arranged in parallel with a region for intrinsic information to be read out or written in by the head. As a consequence, at least one surface of the disc is used for storing reference information.
However, it has been found out that when the disc rotates at a high rotational speed, the above-described parallel control is not necessarily needed and that when the position of the read/write head is correctly controlled only once at every rotation of the disc, the correct positioning condition can be properly maintained. As a result, the region on the disc required for the storage of reference information can be reduced in area, so that it now becomes possible to store reference information on the surface of the disc upon which actual data information is stored; that is, on the so-called data surface of the disc. In this case, the reference information regions must be inserted into data information regions, and hence it is better that the reference information regions be reduced in area in order to attain a high data storage density.
However, in the case of the prior art closed loop control system of the type described, in order to correctly evaluate the peak values of respective analog pulses by separating the analog pulses resulting from the reading of reference information, domains which are magnetized in the N and S poles cannot be reduced in area below a certain limited area. Furthermore, in general, reference information must be repeatedly stored. Therefore, each reference information storage region must be considerably extended.